1. Field of the Invention
The present invention relates to a surface emission type semiconductor laser adapted to emit a laser beam in a direction perpendicular to a substrate.
2. Description of the Related Art
A surface emission type laser including a resonator disposed in a direction perpendicular to the substrate thereof is disclosed in Lectures of the 50-th Meeting of Applied Physics in Japan (Sep. 27, 1989), Vol. 3, pp. 909, 29a-ZG-7. In accordance with the prior art, as shown in FIG. 12, there is first provided an n-type GaAs substrate 602 on which an n-type AlGaAs/AlAs multi-layer film 603, an n-type AlGaAs cladding layer 604, a p-type GaAs active layer 605 and a p-type AlGaAs cladding layer 606 are sequentially grown and formed. The multi-layered structure is then etched while leaving a column-like part at the top thereof. The remaining column-like part is enclosed by a buried layer which is formed by sequentially growing a p-type layer 607, n-type layer 608, p-type layer 609 and p-type layer 610 all of which are of AlGaAs in liquid phase epitaxy method. Thereafter, a multi-layer dielectric film 611 is deposited on the cap layer of p-type AlGaAs 610 at the top thereof. Finally, p- and n-type ohmic electrodes 612 and 601 are formed respectively on the top and bottom end faces of the structure thus formed. In such a manner, a surface emission type semiconductor laser will be completed.
The buried layer (607-608) formed in the above manner defines a p-n junction which is used as means for preventing current from leaking to layer sections other than the active layer section.
However, by using such a p-n junction, it is difficult to provide a sufficient current restriction; and it cannot suppress any reactive current perfectly. Due to generation of heat in the component, therefore, the surface emission type semiconductor laser constructed in accordance with the prior art is impractical in that it is difficult to perform a continuous generating drive in room temperature. It is thus important to restrict the reactive current in the surface emission type semiconductor laser.
Where the buried layer is of a multi-layered structure to form a p-n junction as in the prior art, the p-n interface in the buried layer should be positioned in consideration of a position of the interface between each of the adjacent column-like grown layers. It is difficult to control the thickness of each layer in the multi-layered structure. It is therefore very difficult to consistently produce surface emission type semiconductor lasers.
If a buried layer is formed around the column by the liquid phase epitaxy method as in the prior art, there is a high risk of breaking-off of the column-like part, leading to a reduced yield. The prior art was thus subject to a structural limitation in improving its characteristics.
The prior art raises further problems even when it is applied to various other devices such as laser printers and the like.
For example, laser printers can have an increased freedom of design as in simplifying the optical system or in decreasing the optical path, since the source of light (semiconductor laser and so on) has a relatively large size of light spot equal to several tens .mu.m and if a light emitting element having an increased intensity of light emission is used in the laser printers.
With the surface emission type semiconductor laser constructed according to the prior art, the optical resonator is entirely buried in a material having a refractive index higher than that of the resonator. Light rays are mainly guided in the vertical direction. As a result, a spot of light emission in the basic generation mode will have a diameter equal to about 2 .mu.m even if the shape of the resonator is modified in the horizontal direction.
It has been proposed that the light spots be located close to each other up to about 2 .mu.m and that a plurality of light sources be used to increase the size of a spot. From the standpoint of reproductiveness and yield, however, it is very difficult with the prior art to bury a plurality of resonators spaced away from one another by several microns to using the LPE method. Even if such a burying can be successfully carried out, the spots cannot be united into a single spot since the transverse leakage of light is little.
It is also necessary that a plurality of light spots are formed into a single beam of light and that the laser beams each consisted of plural spots are in phase to increase the intensity of light emission. However, the prior art could not produce a surface emission type semiconductor laser which emits a plurality of laser beams close to one another up to a distance by which one of the laser beams are influenced by the other, in order to synchronize the laser beams in phase.
In such a type of surface emission type semiconductor lasers, it is very important to set a plane of polarization in the laser beam in a particular direction. The plane of polarization is one in which the laser beam oscillates. The laser beam has a property that it proceeds while the plane of polarization is being maintained in one direction.
It is known in the art that when a laser beam is reflected by a mirror, its reflectivity depends on an angle included between the plane of polarization and the mirror. Since the laser beam has such a property, the direction of polarization plane in the laser beam becomes very important in designing an instrument utilizing the laser beam, such as laser printer. For example, when a laser beam is to be reflected by a mirror to provide a desired level of light intensity in the reflected laser beam, it is required to adjust the position of an element so that the direction of polarization plane can be set. If a plurality of elements have different directions of polarization plane in laser beams, it is extremely troublesome to adjust the position of each element. In addition, when one element includes a plurality of semiconductor lasers and if the directions of polarization plane in the laser beams are different from one another, the reflectivity in the mirror varies from one laser beam to another to provide the resultant intensities in the reflected beams which are different from one another. In the other fields in which a laser beam is caused to pass through a polarizing filter or the like, such as optical communication, it is required that only a laser beam having a particular direction of polarization plane is caused to pass through the polarizing filter. It is very important herein that the direction of polarization plane is set in a particular direction.
However, the actual measurements of surface emission type semiconductor lasers relating to the direction of polarization plane in the laser beam showed that the direction of polarization plane was different from one element to another and that when a plurality of lasers are arranged within a single element, there was a variability on the directions of polarization plane even in the laser beams which were emitted from the one and same element.
From our study, it has been found that the direction of polarization plane depends on the cross-sectional configuration of a resonator parallel to a semiconductor substrate. In the plane of polarization of the prior art, a buried layer was formed to surround a resonator by the use of LPE (Liquid Phase Epitaxy) process after the resonator had been formed into a column-like configuration, as described. In the LPE process, it cannot be avoided to create an undesirable phenomenon which is called "melt-back". The "melt-back" phenomenon causes the resonator to deform since the sides of the column-like resonator are fused by contact with the high-temperature molten material. The fusion of resonator sides due to the "melt-back" is very unstable, depending on the surface state, crystal orientation and other factors. Therefore, even if a resonator having its desirable cross-sectional configuration was formed by an etching technique, the cross-sectional configuration thereof would be deformed by the "melt-back" when it was subjected to the LPE process. Consequently, the direction of polarization plane in the laser beam emitted from such a resonator could not be also set in a desired direction.